1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular, an image forming apparatus for forming an image by scanning a surface to be scanned as an image support body such as a photosensitive body or an electrostatic recording body with a modulated light flux and also to an image forming method.
2. Related Background Art
In a conventional image forming apparatus using an electrophotographic system in which a visual image is formed by scanning of charging, exposure, and development, as a means of forming an electrostatic latent image after primary charging to the electrophotographic photosensitive body as the image bearing body, a means for performing exposure using a semiconductor laser becomes widely practical use. In the means for forming the electrostatic latent image, a laser chip composed of a laser diode and a photo diode sensor is used. An output signal from the photo diode sensor is fed back to a bias power source of the laser diode and thus automatic control for an amount of bias current is performed to stabilize laser light.
Recently, as the printing speed of the image forming apparatus becomes faster, a means for forming the electrostatic latent image using a multi-laser for simultaneously emitting a plurality of laser lights in one main scanning becomes practical use. For example, even in the multi-laser system using two lasers, the above structure is used, this is composed of two pairs of laser light emitting diodes and photo diode sensors to stabilize laser lights.
On the other hand, various image signal processing techniques are used for image improvement. As one of these techniques, in the case where a digital image signal is binarized to form the image, a pulse width modulation (PWM) method is proposed such that the digital image signal is converted into an analog signal and then compared with a periodic pattern signal such as a triangle wave to produce a binary signal pulse-width-modulated. The invention in that the PWM is used for a multi-beam printer type laser printer is disclosed in Japanese Patent Application Laid-open No. 08-317157. According to the invention disclosed therein, in order to prevent an unevenness density due to an individual difference of respective lasers in the multi-beam, a pattern signal of the respective lasers is corrected with the pulse width modulation. That is, there is the case where the respective beams are individually pulse-width-modulated in response to respective beam properties. Thus, light intensities of the respective lasers are kept and thus light portion potentials in respective laser scans are equaled to suppress the density unevenness.
However, in the multi-beam, even if no individual difference of the pair of lasers is produced, there is a problem that a half tone density is different. This is a new problem that, even in the case of the same image pattern, if a write starting position in a sub scanning direction is shifted by one, a half tone image density becomes different. It is considered that such a phenomenon is caused by non-linearity of a curve (E−V curve) between a light intensity E and a potential V in a photosensitive body. For example, when a light strength is given by I and an exposure time is given by t, the light intensity E is given by an equation E=I×t. Even if the photosensitive body is provided with the same light intensity E, when the light strength I or the exposure time t is changed, there is the case where the sensitivity is different and thus the potential is changed. Thus, the density difference is produced. This is called a reciprocity failure. In relation to this reciprocity failure, an example that the sensitivity is improved when the photosensitive body is irradiated with light having a low intensity for a plurality of times, is reported in Japanese Patent Application Laid-open No. 04-51043.
Hereinafter, an example of a half tone density difference which is produced by the reciprocity failure in the multi-beam will be shown.
FIG. 16 is a schematic view showing a half tone of 2 dots and 2 spaces in the case where beams “A” and “B” are simultaneously irradiated onto a paper. A pair of lasers are defined as the beams “A” and “B”. The beam “A” corresponds to a first line as a head of a write position of a paper and the beam “B” corresponds to a second line. After that, the beams “A” and “B” alternately correspond to respective lines. Thus, the beam “A” corresponds to an odd line and the beam “B” corresponds to an even line. The beams “A” and “B” are simultaneously turned ON in a first polygon scanning to scan image data in horizontal lines of 2 dots, and then simultaneously turned OFF in next polygon scanning to provide 2 spaces. Thus, the beams “A” and “B” are repeatedly simultaneous-turned ON and OFF to become a half tone. Note that, in FIG. 16, a pair of lasers in the polygon scanning are separated from each other by dashed lines.
FIG. 17 is a schematic view showing a half tone of 2 dots and 2 spaces in the case where the beams “A” and “B” are alternately irradiated onto a paper. In a first polygon scanning, the beam “A” is turned OFF and the beam “B” is turned ON. Thus, 1 space is provided and image data is scanned in a horizontal line of 1 dot. In the next polygon scanning, the beam “A” is turned ON and the beam “B” is turned OFF. Thus, image data is scanned in a horizontal line of 1 dot and 1 space is provided. Therefore, When a half tone of 1 space and 1 dot and a half tone of 1 dot and 1 space are repeated in succession, the half tone of 2 dots and 2 spaces which is shifted by one line is obtained.
Densities of the half tones of 2 dots and 2 spaces shown in FIGS. 14 and 15 are compared. In the half tone of 2 dots and 2 spaces in the case where two lasers are simultaneously irradiated in the main scanning line direction, as shown in FIG. 14, the density is 1.15. On the other hand, in the half tone of 2 dots and 2 spaces in the case where two lasers are alternately irradiated in the main scanning line direction, the density is 1.21. Thus, the density in the case of the laser simultaneous irradiation is lower than that in the case of the laser alternate irradiation.
In order to find this cause, whether a difference in a light intensity is produced or not is examined. It is considered that the respective lasers are interfered by thermal and electrical crosstalk between the lasers and thus the light intensity is decreased at the simultaneous irradiation. Thus, the laser light intensities in the cases of the simultaneous irradiation and the single shot irradiation are measured and compared.
FIG. 18 shows a measurement value of the light intensity by a pin photo diode in the case where the beam “A” is scanned with a single shot. FIG. 19 shows a measurement value of the light intensity by the pin photo diode in the case where the beam “B” is scanned with a single shot. FIG. 20 shows a measurement value of the light intensity by the pin photo diode in the case where the beams “A” and “B” are simultaneously emitted (turned on) and scanned. In this light intensity measurement, when the light intensity of the beam “A” as shown in FIG. 18 and that of the beam “B” as shown in FIG. 19 are summed, the summed light intensity agrees with the light intensity in the case of the simultaneous irradiation as shown in FIG. 20. As a result, it is found that even if the simultaneous turning on is performed, the light intensities of the multi-beam are stable and not decreased.
Next, whether a difference in a potential of a photosensitive body is produced or not is examined. A spot diameter used here is not sufficiently small. Thus, it is expected that two laser spots are overlapped with each other and a potential in the overlapped portion is different. As the conditions, the spot size of the beam “A” is equal to that of the beam “B” and is 70 μm in the main scanning direction and 70 μm in the sub scanning direction. In the image forming apparatus of 1200 dpi, a size of 1 pixel is 21 μm.
FIG. 21 is a concept view in the case where the light intensity distribution at the simultaneous exposure is converted into the potential distribution through an E−V curve. The beam “A” and the beam “B” are overlapped with each other to become the light intensity of the multi-beam and thus irradiated into the photosensitive body. The light intensity distribution is converted into the potential through the E−V curve. A point remarked here is an overlapped portion of the spots. The beam having the total light intensity is irradiated into the photosensitive body, and simultaneously holes are produced to determine the potential distribution.
FIG. 22 is a concept view in the case where the light intensity distribution at the separate exposure is converted into the potential distribution through the E−V curve. An arrow (1) represents a path in the case where the first beam “A” with a predetermined intensity is irradiated into the photosensitive body to produce holes, and thus a first potential distribution is determined. Also, an arrow (2) represents a path in the case where the next beam “B” is irradiated into the photosensitive body to produce holes, and thus a second potential distribution is determined.
When the case of FIG. 21 is compared with the case of FIG. 22, the total light intensity is the same in the overlapped portion of the spots. However, when the simultaneous irradiation is made, the photosensitive body is exposed to strong light by one shot and thus the potential distribution is determined by one time. On the other hand, even when the photosensitive body is separately irradiated with weak light, the E−V curve becomes a non linear convex form downward. Therefore, the potential can be sufficiently decreased and thus the E−V curve becomes the superposition of two potential distributions. Note that, since the above E−V curve for changing the light intensity to the potential is a curve obtained at solid exposure, it is not exact to apply this curve to the case of the half tone of 2 dots and 2 spaces. Therefore, the E−V curve obtained in the case of 2 dots and 2 spaces is actually measured, and then whether a difference between the simultaneous exposure and the separate exposure is produced or not is examined with respect to the photosensitive body having the E−V curve which becomes the non linear convex form downward.
FIG. 23 shows results obtained by measuring a surface potential of the photosensitive body by changing the light intensity in the cases of 2 dots and 2 spaces by the simultaneous irradiation and the separate irradiation using the beams “A” and “B” in the multi-beam. As can be seen from the graph shown in FIG. 21, a potential curve corresponding to the light intensity in 2 dots and 2 spaces by the simultaneous irradiation is always higher than that in 2 dots and 2 spaces by the separate irradiation and thus the sensitivity in the simultaneous irradiation is deteriorated relatively to the separate irradiation. Concretely, a light intensity set value in the image forming apparatus is generally 3.0 mJ/m2. Then, when two beams are simultaneously irradiated into the photosensitive body, as shown in FIG. 14, a potential of −265 V was obtained. On the other hand, when two beams are separately irradiated into the photosensitive body, as shown in FIG. 15, a potential of −250 V was obtained. Here, since a density is a reverse phenomenon, the density in the potential of −265 V becomes lower than that in the potential of −250 V. As described above, a difference between these potentials corresponds to that between the densities of 1.15 and 1.21. Therefore, when the density in the separate irradiation is adjusted to that in the simultaneous irradiation, it is required that the light intensity in the separate irradiation is decreased to about ⅞ and thus the light intensity set value is set to be 2.6 mJ/m2.
As described above, in the case of the multi-beam, even if the same light intensity is set in the cases of simultaneous exposure and the separate exposure using two beams, it is found that the potential becomes higher and the sensitivity is deteriorated in the simultaneous exposure relatively to the separate exposure by the reciprocity failure of the photosensitive body. That is, in the overlapped portion of the multi-beam, when the multi-beam is simultaneously emitted (turned on), the light intensities of respective beams are superimposed and then the photosensitive body is irradiated with this multi-beam at once. On the other hand, in the overlapped portion of the multi-beam, the multi-beam is separately emitted (turned on), the photosensitive body is separately irradiated with the respective beams. At this time, a write position in a half tone image is shifted by only one line. However, there is a problem that the sensitivity is deteriorated and the difference in the density is produced in the former relatively to the latter.